<template>
  <canvas id="canvas"></canvas>
</template>
<script>
  export default {
    mounted() {
      this.$nextTick(_=>{
        this.init();
      })
    },
    methods: {
      init() {
        var canvas = document.getElementById('canvas'),
          ctx = canvas.getContext('2d'),
          w = canvas.width = window.innerWidth,
          h = canvas.height = window.innerHeight,
          hue = 217,
          stars = [],
          count = 0,
          maxStars = 1300; //星星数量
          ctx.fillStyle = 'rgba(255, 255, 255, 0)';

        var canvas2 = document.createElement('canvas'),
          ctx2 = canvas2.getContext('2d');
        canvas2.width = 100;
        canvas2.height = 100;
        var half = canvas2.width / 2,
          gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
        gradient2.addColorStop(0.025, '#CCC');
        gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
        gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
        gradient2.addColorStop(1, 'transparent');

        ctx2.fillStyle = gradient2;
        ctx2.beginPath();
        ctx2.arc(half, half, half, 0, Math.PI * 2);
        ctx2.fill();

        function random(min, max) {
          if (arguments.length < 2) {
            max = min;
            min = 0;
          }

          if (min > max) {
            var hold = max;
            max = min;
            min = hold;
          }

          return Math.floor(Math.random() * (max - min + 1)) + min;
        }

        function maxOrbit(x, y) {
          var max = Math.max(x, y),
            diameter = Math.round(Math.sqrt(max * max + max * max));
          return diameter / 2;
          //星星移动范围，值越大范围越小，
        }

        var Star = function () {

          this.orbitRadius = random(maxOrbit(w, h));
          this.radius = random(60, this.orbitRadius) / 8;
          //星星大小
          this.orbitX = w / 2;
          this.orbitY = h / 2;
          this.timePassed = random(0, maxStars);
          this.speed = random(this.orbitRadius) / 50000;
          //星星移动速度
          this.alpha = random(2, 10) / 10;

          count++;
          stars[count] = this;
        }

        Star.prototype.draw = function () {
          var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
            y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
            twinkle = random(10);

          if (twinkle === 1 && this.alpha > 0) {
            this.alpha -= 0.05;
          } else if (twinkle === 2 && this.alpha < 1) {
            this.alpha += 0.05;
          }

          ctx.globalAlpha = this.alpha;
          ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
          this.timePassed += this.speed;
        }

        for (var i = 0; i < maxStars; i++) {
          new Star();
        }

        function animation() {
          ctx.globalCompositeOperation = 'source-over';
          ctx.globalAlpha = 0.5; //尾巴
          ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
          ctx.fillRect(0, 0, w, h)

          ctx.globalCompositeOperation = 'lighter';
          for (var i = 1, l = stars.length; i < l; i++) {
            stars[i].draw();
          };

          window.requestAnimationFrame(animation);
        }

        animation();
      }
    }
  }

</script>
